Magnetic tape

ABSTRACT

Provided is a magnetic tape in which an Ra measured regarding a surface of a magnetic layer is equal to or smaller than 1.8 nm, Int(110)/Int(114) of a hexagonal ferrite crystal structure obtained by an XRD analysis of the magnetic layer by using an In-Plane method is 0.5 to 4.0, a vertical squareness ratio of the magnetic tape is 0.65 to 1.00, full widths at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer before and after performing a vacuum heating with respect to the magnetic tape are greater than 0 nm and equal to or smaller than 10.0 nm, and a difference between the spacings measured by optical interferometry regarding the surface of the back coating layer before and after performing the vacuum heating is greater than 0 nm and equal to or smaller than 8.0 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. 119 to Japanese Patent Application No. 2017-140024 filed on Jul. 19, 2017. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic tape.

2. Description of the Related Art

Magnetic recording media are divided into tape-shaped magnetic recording media and disk-shaped magnetic recording media, and tape-shaped magnetic recording media, that is, magnetic tapes are mainly used for storage such as data back-up.

As the magnetic tapes, JP1989-60819A (JP-H01-60819A) discloses a magnetic tape including a back coating layer (described as a “back layer” in JP1989-60819A (JP-H01-60819A)) on a surface side of a non-magnetic support opposite to a surface side provided with a magnetic layer.

SUMMARY OF THE INVENTION

In addition, in recent years, it is necessary that surface smoothness of a magnetic layer is increased in a magnetic tape. This is because an increase in surface smoothness of a magnetic layer causes improvement of electromagnetic conversion characteristics.

However, the inventors have made studies regarding a magnetic tape including a back coating layer and found that, particularly, in a magnetic tape in which surface smoothness of a magnetic layer is increased so that a center line average surface roughness Ra measured regarding a surface of the magnetic layer is equal to or smaller than 1.8 nm, a phenomenon in which an edge part of the magnetic tape is damaged (hereinafter, referred to as “edge damage”) significantly occurs, after causing the magnetic tape to run in a drive. In regards to this point, more specifically, the recording of information on the magnetic tape and/or reproducing the recorded information is performed by setting a magnetic tape cartridge in a drive, causing the magnetic tape to run in the drive, and causing the magnetic head mounted on the drive to come into contact with the surface of the magnetic layer for sliding. Since the magnetic tape is accommodated in the magnetic tape cartridge in a state of being wound around the reel, sending of the magnetic tape from the reel and winding thereof are performed during the running of the magnetic tape in the drive. In a case where disordered winding occurs at the time of this winding, an edge of the magnetic tape hits against a flange or the like of a reel and the edge damage may occur. This edge damage may cause an increase in the number of errors at the time of recording and/or a deterioration of miming stability. Accordingly, it is necessary that the disordered winding is prevented to reduce the edge damage.

Therefore, an aspect of the invention provides for a magnetic tape in which a center line average surface roughness Ra measured regarding a surface of the magnetic layer is equal to or smaller than 1.8 nm and occurrence of edge damage is prevented.

According to one aspect of the invention, there is provided a magnetic tape comprising: a non-magnetic support; a magnetic layer including ferromagnetic powder, non-magnetic powder, and a binding agent on one surface side of the non-magnetic support; and a back coating layer including non-magnetic powder and a binding agent on the other surface side of the non-magnetic support, in which the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, a center line average surface roughness Ra measured regarding a surface of the magnetic layer (hereinafter, also referred to as a “magnetic layer surface roughness Ra”) is equal to or smaller than 1.8 nm, an intensity ratio (Int(110)/Int(114); hereinafter, also referred to as “X-ray diffraction (XRD) intensity ratio) of a peak intensity Int(110) of a diffraction peak of a (110) plane with respect to a peak intensity Int(114) of a diffraction peak of a (114) plane of a hexagonal ferrite crystal structure obtained by an X-ray diffraction analysis of the magnetic layer by using an In-Plane method is 0.5 to 4.0, a vertical squareness ratio of the magnetic tape is 0.65 to 1.00, the back coating layer includes fatty acid ester, a full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer before performing a vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 10.0 nm, a full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 10.0 nm, and a difference (S_(after)−S_(before)) between a spacing S_(after) measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape and a spacing S_(before) measured by optical interferometry regarding the surface of the back coating layer before performing the vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 8.0 nm.

In one aspect, the center line average surface roughness Ra measured regarding the surface of the magnetic layer may be 1.2 nm to 1.8 nm.

In one aspect, the vertical squareness ratio of the magnetic tape may be 0.65 to 0.90.

In one aspect, the full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer before performing the vacuum heating with respect to the magnetic tape may be greater than 0 nm and equal to or smaller than 8 5 nm.

In one aspect, the full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape may be greater than 0 nm and equal to or smaller than 8.5 nm.

In one aspect, the difference (S_(after)−S_(before)) between the spacing S_(after) measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape and the spacing S_(before) measured by optical interferometry regarding the surface of the back coating layer before performing the vacuum heating with respect to the magnetic tape may be greater than 0 nm and equal to or smaller than 6.0 nm.

In one aspect, the non-magnetic powder included in the back coating layer may be one or more kinds of non-magnetic powder selected from the group consisting of inorganic powder and carbon black.

In one aspect, a content of the inorganic powder in the back coating layer may be greater than 50.0 parts by mass and equal to or smaller than 100.0 parts by mass with respect to 100.0 parts by mass of a total amount of the non-magnetic powder included in the back coating layer.

In one aspect, the magnetic tape may further comprise a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer.

According to one aspect of the invention, it is possible to provide a magnetic tape which has high surface smoothness of a magnetic layer and in which occurrence of edge damage is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of a vibration imparting device used in examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One aspect of the invention relates to a magnetic tape including: a non-magnetic support; a magnetic layer including ferromagnetic powder, non-magnetic powder, and a binding agent on one surface side of the non-magnetic support; and a back coating layer including non-magnetic powder and a binding agent on the other surface side of the non-magnetic support, in which the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, a center line average surface roughness Ra measured regarding a surface of the magnetic layer is equal to or smaller than 1.8 nm, an intensity ratio (Int(110)/Int(114)) of a peak intensity Int(110) of a diffraction peak of a (110) plane with respect to a peak intensity Int(114) of a diffraction peak of a (114) plane of a hexagonal ferrite crystal structure obtained by an X-ray diffraction analysis of the magnetic layer by using an In-Plane method is 0.5 to 4.0, a vertical squareness ratio of the magnetic tape is 0.65 to 1.00, the back coating layer includes fatty acid ester, a full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer before performing a vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 10.0 nm, a full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 10.0 nm, and a difference (S_(after)−S_(before)) between a spacing S_(after) measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape and a spacing S_(before) measured by optical interferometry regarding the surface of the back coating layer before performing the vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 8.0 nm.

Hereinafter, the magnetic tape will be described more specifically. The following description includes a surmise of the inventors. The invention is not limited to such a surmise. In addition, hereinafter, exemplary description may be made with reference to the drawings. However, the invention is not limited to the exemplified aspects.

In the magnetic tape, even in a case where the magnetic layer surface roughness Ra is equal to or smaller than 1.8 nm, occurrence of edge damage can be prevented. In regards to this point, the inventors have considered as follows.

The inventors have considered that a reason for which the edge damage significantly occurs in a magnetic tape having the magnetic layer surface roughness Ra equal to or smaller than 1.8 nm, is because an unstable contact state between the surface of the magnetic layer and the surface of the back coating layer, in a case of the winding, due to an increase in surface smoothness of the magnetic layer. With respect to this, the inventors have considered that, in the magnetic tape, the XRD intensity ratio, the vertical squareness ratio, and various values measured regarding the surface of the back coating layer set to be in the respective ranges described above contribute to an increase in stability of the contact state between the surface of the magnetic layer and the surfaces of the back coating layer, thereby preventing occurrence of the edge damage occurred due to the disordered winding. This point will be described later in detail.

In the invention and the specification, the “surface of the magnetic layer” of the magnetic tape is identical to the surface of the magnetic tape on the magnetic layer side. The “surface of the back coating layer” is identical to the surface of the magnetic tape on the back coating layer side. In the invention and the specification, the “ferromagnetic hexagonal ferrite powder” means an aggregate of a plurality of ferromagnetic hexagonal ferrite particles. The ferromagnetic hexagonal ferrite particles are ferromagnetic particles having a hexagonal ferrite crystal structure. Hereinafter, particles (ferromagnetic hexagonal ferrite particles) configuring the ferromagnetic hexagonal ferrite powder are also referred to as simply “particles”. The “aggregate” not only includes an aspect in which particles configuring the aggregate are directly in contact with each other, but also includes an aspect in which a binding agent, an additive, or the like is sandwiched between the particles. The points described above are also applied to various powders such as non-magnetic powder of the invention and the specification, in the same manner.

In the invention and the specification, the description regarding directions and angles (for example, vertical, orthogonal, parallel, and the like) includes a range of errors allowed in the technical field of the invention, unless otherwise noted. For example, the range of errors means a range of less than ±10° from an exact angle, and is preferably within ±5° and more preferably within ±3° from an exact angle.

In addition, hereinafter, the full width at half maximum (FWHM) of spacing distribution measured by optical interferometry regarding the surface of the back coating layer before performing the vacuum heating with respect to the magnetic tape is also referred to as “FWHM_(before) of the back coating layer” or “FWHM_(before)”, and the full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape is also referred to as “FWHM_(after) of the back coating layer” or “FWHM_(after)”. In addition, the difference between a spacing S_(after) measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape and a spacing S_(before) measured by optical interferometry regarding the surface of the back coating layer before performing the vacuum heating with respect to the magnetic tape is also referred to as the “difference (S_(after)−S_(before)) of the back coating layer” or the “difference (S_(after)−S_(before))”.

Magnetic Layer Surface Roughness Ra

The magnetic layer surface roughness Ra measured regarding the surface of the magnetic layer of the magnetic tape (magnetic layer surface roughness Ra) is equal to or smaller than 1.8 nm. It is preferable that the magnetic layer surface roughness Ra is equal to or smaller than 1.8 nm, from a viewpoint of improving electromagnetic conversion characteristics. However, as described above, the edge damage significantly occurs in the magnetic tape having increased surface smoothness of the magnetic layer so that the magnetic layer surface roughness Ra becomes equal to or smaller than 1.8 nm. With respect to this, in the magnetic tape, by setting the XRD intensity ratio, the vertical squareness ratio, the FWHM_(before) and the FWHM_(after) measured by optical interferometry regarding the surface of the back coating layer, and the difference (S_(after)−S_(before)) to be in the respective ranges described above, occurrence of the edge damage can be prevented. From a viewpoint of further improving electromagnetic conversion characteristics, low magnetic layer surface roughness Ra is preferable. From this viewpoint, the magnetic layer surface roughness Ra can be equal to or smaller than 1.7 nm or equal to or smaller than 1.6 mm In addition, the magnetic layer surface roughness Ra can be, for example, equal to or greater than 1.2 nm or equal to or greater than 1.3 nm. However, low magnetic layer surface roughness Ra is preferable, from a viewpoint of improving electromagnetic conversion characteristics, and thus, the magnetic layer surface roughness Ra may be lower than the value exemplified above.

The center line average surface roughness Ra measured regarding the surface of the magnetic layer of the magnetic tape in the invention and the specification is a value measured with an atomic force microscope (AFM) in a region having an area of 40 μm×40 μm of the surface of the magnetic layer. As an example of the measurement conditions, the following measurement conditions can be used. The magnetic layer surface roughness Ra shown in examples which will be described later is a value obtained by the measurement under the following measurement conditions.

The measurement is performed regarding the region of 40 μm×40 μm of the area of the surface of the magnetic layer of the magnetic tape with an AFM (Nanoscope 4 manufactured by Veeco Instruments, Inc.) in a tapping mode. RTESP-300 manufactured by BRUKER is used as a probe, a scan speed (probe movement speed) is set as 40 μm/sec, and a resolution is set as 512 pixel×512 pixel.

The magnetic layer surface roughness Ra can be controlled by a well-known method. For example, the magnetic layer surface roughness Ra can be changed in accordance with the size of various powders included in the magnetic layer (for example, ferromagnetic hexagonal ferrite powder, non-magnetic powder, and the like) or manufacturing conditions of the magnetic tape. Thus, by adjusting these, it is possible to obtain the magnetic tape having the magnetic layer surface roughness Ra equal to or smaller than 1.8 nm.

XRD Intensity Ratio and Vertical Squareness Ratio

Next, the XRD intensity ratio and the vertical squareness ratio will be described. The magnetic tape of the magnetic tape device includes ferromagnetic hexagonal ferrite powder and non-magnetic powder in the magnetic layer. The non-magnetic powder in the magnetic layer can preferably function as an abrasive or a projection formation agent, as will be described later in detail. However, it is thought that, in a case where the particles of non-magnetic powder (non-magnetic particles) present in the surface of the magnetic layer and/or the vicinity of the surface of the magnetic layer do not suitably sink into the magnetic layer with a force received from the magnetic head, at the time of the sliding of the surface of the magnetic layer and the magnetic head, chipping of the magnetic head (head chipping) may occur due to the contact with the particles of the non-magnetic powder. On the other hand, it is thought that, in a case where the particles of the non-magnetic powder excessively sink into the magnetic layer, at the time of the sliding of the surface of the magnetic layer and the magnetic head, the area of the contact between the surface of the magnetic layer and the magnetic head (real contact) increases, a force applied to the surface of the magnetic layer from the magnetic head at the time of the sliding becomes strong, and the surface of the magnetic layer is damaged, thereby causing the chipping of the surface of the magnetic layer.

The inventors have surmised that scraps generated due to the head chipping and scraps generated due to the chipping of the surface of the magnetic layer described above are interposed between the surface of the magnetic layer and the surface of the back coating layer, thereby decreasing stability of a contact state between the surface of the magnetic layer and the surface of the back coating layer at the time of winding.

In regards to this point, the inventors have surmised that, particles which supports the particles of the non-magnetic powder put into the magnetic layer and affects a degree of the sinking (hereinafter, also referred to as “former particles”) and particles which are considered not to affect or slightly affects the sinking (hereinafter, also referred to as “latter particles”) are included in the particles configuring the ferromagnetic hexagonal ferrite powder included in the magnetic layer. It is considered that the latter particles are, for example, fine particles generated due to partial chipping of particles due to a dispersion process performed at the time of preparing a magnetic layer forming composition.

The inventors have thought that, in the particles configuring the ferromagnetic hexagonal ferrite powder (ferromagnetic hexagonal ferrite particles) included in the magnetic layer, the former particles are particles causing the diffraction peak in the X-ray diffraction analysis using the In-Plane method, and since the latter particles are fine, the latter particles do not or hardly affect the diffraction peak. Accordingly, it is surmised that it is possible to control a presence state of the ferromagnetic hexagonal ferrite particles which supports the particles of the non-magnetic powder put into the magnetic layer and affects a degree of the sinking, in the magnetic layer, based on the intensity of the diffraction peak caused by the X-ray diffraction analysis of the magnetic layer using the In-Plane method, and as a result, it is possible to control a degree of the sinking of the particles of the non-magnetic powder. Specifically, the inventors have surmised that, as the value of the XRD intensity ratio is small, the particles of the non-magnetic powder easily sink, and as the value thereof is great, the particles thereof hardly sink. The inventors have thought that, the sinking of the particles of the non-magnetic powder can be suitably controlled to a degree that, the head chipping and the chipping of the surface of the magnetic layer can be prevented, by setting the XRD intensity ratio to be 0.5 to 4.0. The inventors have surmised that, this causes preventing a decrease in stability of the contact state between the surface of the magnetic layer and the surface of the back coating layer at the time of the winding, due to the scraps generated due to the head chipping and the scraps generated due to the surface of the magnetic layer which are interposed between the surface of the magnetic layer and the surface of the back coating layer.

Meanwhile, the vertical squareness ratio is a ratio of residual magnetization with respect to saturation magnetization measured in a direction vertical to the surface of the magnetic layer and this value decreases, as a value of the residual magnetization decreases. It is surmised that, since the latter particles are fine and hardly hold magnetization, as a large amount of the latter particles is included in the magnetic layer, the vertical squareness ratio tends to decrease. Accordingly, the inventors have thought that the vertical squareness ratio may be an index for the amount of the latter particles (fine particles) present in the magnetic layer. In addition, the inventors have surmised that, as a large amount of the fine particles is included in the magnetic layer, hardness of the magnetic layer is decreased, the chipping of the surface of the magnetic layer easily occurs due to the contact with the magnetic head and the like, the scraps generated due to the chipping are interposed between the surface of the magnetic layer and the surface of the back coating layer, and accordingly, stability of the contact state between the surface of the magnetic layer and the surface of the back coating layer at the time of the winding decreases. With respect to this, the inventors have thought that, in the magnetic layer having the vertical squareness ratio of 0.65 to 1.00, the chipping of the surface of the magnetic layer which is hardly performed due to the decreased presence amount of the latter particles (fine particles) contributes to an increase in stability of the contact state between the surface of the magnetic layer and the surface of the back coating layer at the time of the winding.

As described above, the invention have surmised that the XRD intensity ratio of 0.5 to 4.0 and the vertical squareness ratio of 0.65 to 1.00 causes an increase in stability of the contact state between the surface of the magnetic layer and the surface of the back coating layer at the time of the winding, thereby contributing to the prevention of occurrence of the edge damage.

However, this is merely a surmise and the invention is not limited thereto.

XRD Intensity Ratio

The XRD intensity ratio is obtained by the X-ray diffraction analysis of the magnetic layer including the ferromagnetic hexagonal ferrite powder by using the In-Plane method. Hereinafter, the X-ray diffraction analysis performed by using the In-Plane method is also referred to as “In-Plane XRD”. The In-Plane XRD is performed by irradiating the surface of the magnetic layer with the X-ray by using a thin film X-ray diffraction device under the following conditions. A measurement direction is a longitudinal direction of the magnetic tape.

Cu ray source used (output of 45 kV, 200 mA)

Scan conditions: 0.05 degree/step, 0.1 degree/min in a range of 20 to 40 degrees

Optical system used: parallel optical system

Measurement method: 2θ_(χ) scan (X-ray incidence angle of 0.25°)

The values of the conditions are set values of the thin film X-ray diffraction device. As the thin film X-ray diffraction device, a well-known device can be used. As an example of the thin film X-ray diffraction device, Smart Lab manufactured by Rigaku Corporation. A sample to be subjected to the In-Plane XRD analysis is a tape sample cut out from the magnetic tape which is a measurement target, and the size and the shape thereof are not limited, as long as the diffraction peak which will be described later can be confirmed.

As a method of the X-ray diffraction analysis, thin film X-ray diffraction and powder X-ray diffraction are used. In the powder X-ray diffraction, the X-ray diffraction of the powder sample is measured, whereas, according to the thin film X-ray diffraction, the X-ray diffraction of a layer or the like formed on a substrate can be measured. The thin film X-ray diffraction is classified into the In-Plane method and an Out-Of-Plane method. The X-ray incidence angle at the time of the measurement is 5.00° to 90.00° in a case of the Out-Of-Plane method, and is generally 0.20° to 0.50°, in a case of the In-Plane method. In the In-Plane XRD of the invention and the specification, the X-ray incidence angle is 0.25° as described above. In the In-Plane method, the X-ray incidence angle is smaller than that in the Out-Of-Plane method, and thus, a depth of penetration of the X-ray is shallow. Accordingly, according to the X-ray diffraction analysis by using the In-Plane method (In-Plane XRD), it is possible to perform the X-ray diffraction analysis of a surface portion of a measurement target sample. Regarding the tape sample, according to the In-Plane XRD, it is possible to perform the X-ray diffraction analysis of the magnetic layer. The XRD intensity ratio is an intensity ratio (Int(110)/Int(114)) of a peak intensity Int(110) of a diffraction peak of a (110) plane with respect to a peak intensity Int(114) of a diffraction peak of a (114) plane of a hexagonal ferrite crystal structure, in X-ray diffraction spectra obtained by the In-Plane XRD. The term Int is used as abbreviation of intensity. In the X-ray diffraction spectra obtained by In-Plane XRD (vertical axis: intensity, horizontal axis: diffraction angle 2θ_(χ) (degree)), the diffraction peak of the (114) plane is a peak at which the 2θ_(χ) is detected at 33 to 36 degrees, and the diffraction peak of the (110) plane is a peak at which the 2θ_(χ) is detected at 29 to 32 degrees.

Among the diffraction plane, the (114) plane having a hexagonal ferrite crystal structure is positioned close to particles of the ferromagnetic hexagonal ferrite powder (ferromagnetic hexagonal ferrite particles) in an easy-magnetization axial direction (c axis direction). In addition the (110) plane having a hexagonal ferrite crystal structure is positioned in a direction orthogonal to the easy-magnetization axial direction.

The inventors have surmised that, in the X-ray diffraction spectra obtained by the In-Plane XRD, as the intensity ratio (Int(110)/Int(114); XRD intensity ratio) of the peak intensity Int(110) of the diffraction peak of a (110) plane with respect to the peak intensity Int(114) of the diffraction peak of the (114) plane of a hexagonal ferrite crystal structure increases, a large number of the former particles present in a state where a direction orthogonal to the easy-magnetization axial direction is closer to a parallel state with respect to the surface of the magnetic layer is present in the magnetic layer, and as the XRD intensity ratio decreases, a small amount of the former particles present in such a state is present in the magnetic layer. It is thought that a state where the XRD intensity ratio is 0.5 to 4.0 means a state where the former particles are suitably aligned in the magnetic layer. It is surmised that this causes an increase in stability of the contact state between the surface of the magnetic layer and the surface of the back coating layer at the time of the winding, thereby contributing to the prevention of occurrence of the edge damage. Details of the surmise are as described above.

The XRD intensity ratio is preferably equal to or smaller than 3.5 and more preferably equal to or smaller than 3.0, from a viewpoint of further preventing occurrence of the edge damage. From the same viewpoint, the XRD intensity ratio is preferably equal to or greater than 0.7 and more preferably equal to or greater than 1.0. The XRD intensity ratio can be, for example, controlled in accordance with process conditions of an alignment process performed in a manufacturing step of the magnetic tape. As the alignment process, the homeotropic alignment process is preferably performed. The homeotropic alignment process can be preferably performed by applying a magnetic field vertically to the surface of a coating layer of a magnetic layer forming composition in a wet state (undried state). As the alignment conditions are reinforced, the value of the XRD intensity ratio tends to increase. As the process conditions of the alignment process, magnetic field strength of the alignment process is used. The process conditions of the alignment process are not particularly limited. The process conditions of the alignment process may be set so as that the XRD intensity ratio of 0.5 to 4.0 can be realized. As an example, the magnetic field strength of the homeotropic alignment process can be 0.10 to 0.80 T or 0.10 to 0.60 T. As dispersibility of the ferromagnetic hexagonal ferrite powder in the magnetic layer forming composition increases, the value of the XRD intensity ratio tends to increase by the homeotropic alignment process.

Vertical Squareness Ratio

The vertical squareness ratio is a squareness ratio measured regarding a magnetic tape in a vertical direction. The “vertical direction” described regarding the squareness ratio is a direction orthogonal to the surface of the magnetic layer. That is, regarding the magnetic tape, the vertical direction is a direction orthogonal to a longitudinal direction of the magnetic tape. The vertical squareness ratio is measured by using an oscillation sample type magnetic-flux meter. Specifically, the vertical squareness ratio of the invention and the specification is a value obtained by sweeping an external magnetic field in the magnetic tape at a measurement temperature of 23° C.±1° C. in the oscillation sample type magnetic-flux meter, under conditions of a maximum external magnetic field of 1194 kA/m (15 kOe) and a scan speed of 4.8 kA/m/sec (60 Oe/sec), and is a value after diamagnetic field correction. The measurement value is obtained as a value obtained by subtracting magnetization of a sample probe of the oscillation sample type magnetic-flux meter as background noise.

The vertical squareness ratio of the magnetic tape is equal to or greater than 0.65. The inventors have surmised that the vertical squareness ratio of the magnetic tape is an index for the presence amount of the latter particles (fine particles) described above. It is thought that, in the magnetic layer in which the vertical squareness ratio of the magnetic tape is equal to or greater than 0.65, the presence amount of such fine particles is small. As described above in detail, the inventors have surmised that this also contributes to prevention of the occurrence of the edge damage.

From a viewpoint of further preventing the occurrence of the edge damage, the vertical squareness ratio is preferably equal to or greater than 0.68, more preferably equal to or greater than 0.70, even more preferably equal to or greater than 0.73, and still more preferably equal to or greater than 0.75. In addition, in principle, a maximum value of the squareness ratio is 1.00. Accordingly, the vertical squareness ratio of the magnetic tape is equal to or smaller than 1.00. The vertical squareness ratio may be, for example, equal to or smaller than 0.95, equal to or smaller than 0.90, equal to or smaller than 0.87, or equal to or smaller than 0.85. It is thought that, a great value of the vertical squareness ratio is preferable, from a viewpoint of decreasing the amount of the fine latter particles in the magnetic layer and further preventing the occurrence of the edge damage. Therefore, the vertical squareness ratio may be greater than the value exemplified above.

The inventors have considered that, in order to set the vertical squareness ratio to be equal to or greater than 0.65, it is preferable to prevent occurrence of fine particles due to partial chipping of the particles in a preparation step of the magnetic layer forming composition. A specific method for preventing the occurrence of chipping will be described later.

FWHM_(before) and FWHM_(after) of the back coating layer and difference (S_(after)−S_(before))

The FWHM_(after) and the spacing S_(after) for obtaining the difference (S_(after)−S_(before)) are values obtained after performing vacuum heating with respect to the magnetic tape. In the invention and the specification, the “vacuum heating” of the magnetic tape is performed by holding the magnetic tape in an environment of a pressure of 200 Pa to 0.01 MPa and at an atmosphere temperature of 70° C. to 90° C. for 24 hours.

In the invention and the specification, the spacing measured by optical interferometry regarding the surface of the back coating layer of the magnetic tape is a value measured by the following method.

In a state where the magnetic tape and a transparent plate-shaped member (for example, glass plate or the like) are overlapped onto each other so that the surface of the back coating layer of the magnetic tape faces the transparent plate-shaped member, a pressing member is pressed against the side of the magnetic tape opposite to the back coating layer side at pressure of 5.05×10⁴ N/m (0.5 atm). In this state, the surface of the back coating layer of the magnetic tape is irradiated with light through the transparent plate-shaped member (irradiation region: 150,000 to 200,000 μm²), and a spacing (distance) between the surface of the back coating layer of the magnetic tape and the surface of the transparent plate-shaped member is acquired based on intensity (for example, contrast of interference fringe image) of interference light generated due to a difference in a light path between reflected light from the surface of the back coating layer of the magnetic tape and reflected light from the surface of the transparent plate-shaped member on the magnetic tape side. The light emitted here is not particularly limited. In a case where the emitted light is light having an emission wavelength over a comparatively wide wavelength range as white light including light having a plurality of wavelengths, a member having a function of selectively cutting light having a specific wavelength or a wavelength other than wavelengths in a specific wavelength range, such as an interference filter, is disposed between the transparent plate-shaped member and a light receiving section which receives reflected light, and light at some wavelengths or in some wavelength ranges of the reflected light is selectively incident to the light receiving section. In a case where the light emitted is light (so-called monochromatic light) having a single luminescence peak, the member described above may not be used. The wavelength of light incident to the light receiving section can be set to be 500 to 700 nm, for example. However, the wavelength of light incident to the light receiving section is not limited to be in the range described above. In addition, the transparent plate-shaped member may be a member having transparency through which emitted light passes, to the extent that the magnetic tape is irradiated with light through this member and interference light is obtained.

The measurement described above can be performed by using a commercially available tape spacing analyzer (TSA) such as Tape Spacing Analyzer manufactured by Micro Physics, Inc., for example. The spacing measurement of the examples was performed by using Tape Spacing Analyzer manufactured by Micro Physics, Inc.

In addition, the full width at half maximum of spacing distribution of the invention and the specification is a full width at half maximum (FWHM), in a case where the interference fringe image obtained by the measurement of the spacing described above is divided into 300,000 points, a spacing of each point (distance between the surface of the back coating layer of the magnetic tape and the surface of the transparent plate-shaped member on the magnetic tape side) is acquired, this spacing is shown with a histogram, and this histogram is fit with Gaussian distribution.

Further, the difference (S_(after)−S_(before)) is a value obtained by subtracting a mode before the vacuum heating from a mode after the vacuum heating of the 300,000 points.

The inventors have surmised as the following (1) and (2) for the various values regarding the back coating layer.

(1) The inventors have thought that the value of the difference (S_(after)−S_(before)) can be an index for a thickness of a liquid film formed of fatty acid ester included in the back coating layer, on the surface of the back coating layer. Details thereof are as described below.

A lubricant is generally divided broadly into a fluid lubricant and a boundary lubricant and fatty acid ester is known as a component which can function as a fluid lubricant. It is considered that fatty acid ester can protect the back coating layer by forming a liquid film on the surface of the back coating layer. In addition, it is thought that, in a case where the magnetic tape is wound around a reel and the surface of the back coating layer is in contact with the surface of the magnetic layer, the liquid film of fatty acid ester can cause a force (meniscus force) of pulling both surfaces between both surfaces due to a meniscus (liquid crosslinking). It is surmised that this contributes to an increase in stability of the contact state between the surface of the back coating layer and the surface of the magnetic layer at the time of the winding. However, it is thought that, in a case where an excessive amount of fatty acid ester is present on the surface of the back coating layer, the meniscus force is strongly applied, and thereby causing the sticking causing a decrease in stability of the contact state.

In regards to this point, it is thought that the difference (S_(after)−S_(before)) of a spacing between a state after the vacuum heating (state in which a liquid film of fatty acid ester is volatilized and removed) and a state before the vacuum heating (state in which the liquid film of fatty acid ester is present) can be an index of the thickness of the liquid film formed of fatty acid ester on the surface of the back coating layer, because the fatty acid ester is a component having properties of volatilizing by vacuum heating. The inventors have surmised that the presence of the liquid film of fatty acid ester on the surface of the back coating layer so that the difference (S_(after)−S_(before)) is greater than 0 nm and equal to or smaller than 8.0 nm, brings the surface of the back coating layer into contact with the surface of the magnetic layer at the time of the winding in a stable contact state, while preventing sticking.

(2) In addition, the invention have thought that the FWHM_(before) and FWHM_(after) which are further set in the ranges in the surface of the back coating layer having the difference (S_(after)−S_(before)) in the range described above, contributes to applying of the meniscus force promoting the prevention of disordered winding between the surface of the back coating layer and the surface of the magnetic layer. In regards to this point, a smaller value of the full width at half maximum of spacing distribution measured by optical interferometry means that a variation in the values of the spacing measured on each part of the surface of the measurement target is small. It is thought that a reason for the variation in the values of the spacing on the surface of the back coating layer of the magnetic tape is a variation in surface shape of the back coating layer (for example, a variation due to a dispersion state of the non-magnetic powder included in the back coating layer) and a variation in thickness of the liquid film formed of fatty acid ester. It is thought that the FWHM_(before) measured before the vacuum heating, that is, in a state where the liquid film of fatty acid ester is present on the surface of the back coating layer becomes great, as the variation in the surface shape of the back coating layer and the variation in the thickness of the liquid film of fatty acid ester are great. Particularly, the FWHM_(before) is greatly affected by the variation in the thickness of the liquid film of fatty acid ester. In contrast, the inventors have surmised that the FWHM_(after) measured after the vacuum heating, that is, in a state where the liquid film of fatty acid ester is removed from the surface of the back coating layer, becomes great, as the variation in the surface shape of the back coating layer is great. That is, it is thought that small FWHM_(before) and FWHM_(after) mean a small variation in the surface shape of the back coating layer and a small variation in the thickness of the liquid film of fatty acid ester on the surface of the back coating layer. The inventors have surmised that an increase in uniformity of the surface shape of the back coating layer and uniformity in the thickness of the liquid film of fatty acid ester so that the FWHM_(before) and FWHM_(after) are greater than 0 nm and equal to or smaller than 10.0 nm, contributes to applying of the meniscus force promoting prevention of the disordered winding between the surface of the back coating layer and the surface of the magnetic layer having the difference (S_(after)−S_(before)) in the range described above.

However, the (1) and (2) are merely surmised and the invention is not limited thereto.

FWHM_(before) and FWHM_(after)

Both of the FWHM_(before) measured before the vacuum heating and the FWHM_(after) measured after the vacuum heating in the surface of the back coating layer of the magnetic tape are greater than 0 nm and equal to or smaller than 10.0 nm. As described above, the inventors have surmised that the FWHM_(before) and FWHM_(after) in the range described above contribute to an increase in stability of the contact state between the surface of the back coating layer and the surface of the magnetic layer at the time of the winding. From a viewpoint of further increasing stability of the contact state, FWHM_(before) and FWHM_(after) preferably equal to or smaller than 9.0 nm, more preferably equal to or smaller 8.5 nm, even more preferably equal to or smaller than 8.0 nm, still preferably equal to or smaller than 7.0 nm, still more preferably equal to or smaller than 6.0 nm, and still even more preferably equal to or smaller than 5.0 nm. The FWHM_(before) and FWHM_(after) can be, for example, equal to or greater than 1.0 nm or equal to or greater than 2.0 nm. From a viewpoint of preventing the occurrence of the edge damage, small values of the FWHM_(before) and FWHM_(after) tend to be preferable. Accordingly, the FWHM_(before) and FWHM_(after) may be lower than the lower limit exemplified above.

The FWHM_(before) measured before the vacuum heating can be decreased mainly by decreasing the variation in the thickness of the liquid film of fatty acid ester. An example of a specific method will be described later. Meanwhile, the FWHM_(after) measured after the vacuum heating can be decreased by decreasing the variation in the surface shape of the back coating layer. In order to realize the decrease described above, it is preferable that dispersibility of the non-magnetic powder in a back coating layer forming composition is improved. The dispersibility can be, for example, adjusted by the kind of the non-magnetic powder used for forming the back coating layer and by a mixing ratio or the like, in a case of including two or more kinds of the non-magnetic powders. An example of a specific method will be described later.

Difference (S_(after)−S_(before))

The difference (S_(after)−S_(before)) of the spacings before and after the vacuum heating measured regarding the surface of the back coating layer of the magnetic tape is greater than 0 nm and equal to or smaller than 8.0 nm. From a viewpoint of further preventing the occurrence of the edge damage, the difference (S_(after)−S_(before)) is preferably equal to or greater than 0.1 nm, more preferably equal to or greater than 1.0 nm, and even more preferably equal to or greater than 1.5 nm. In addition, from the same viewpoint, the difference (S_(after)−S_(before)) is preferably equal to or smaller than 7.0 nm, more preferably equal to or smaller than 6.0 nm, even more preferably equal to or smaller than 5.0 nm, and still more preferably equal to or smaller than 4.0 nm. The difference (S_(after)−S_(before)) can be controlled by the amount of fatty acid ester added to a back coating layer forming composition. As the amount of fatty acid ester added to a back coating layer forming composition increases, the difference (S_(after)−S_(before)) tends to increase.

Hereinafter, the magnetic tape will be described more specifically.

Magnetic Layer

Ferromagnetic Powder

The magnetic layer of the magnetic tape includes ferromagnetic hexagonal ferrite powder as ferromagnetic powder. Regarding the ferromagnetic hexagonal ferrite powder, a magnetoplumbite type (also referred to as an “M type”), a W type, a Y type, and a Z type are known as the crystal structure of the hexagonal ferrite. The ferromagnetic hexagonal ferrite powder included in the magnetic layer may have any crystal structure. In addition, an iron atom and a divalent metal atom are included in the crystal structure of the hexagonal ferrite, as constituent atoms. The divalent metal atom is a metal atom which may become divalent cations as ions, and examples thereof include a barium atom, a strontium atom, an alkali earth metal atom such as calcium atom, and a lead atom. For example, the hexagonal ferrite including a barium atom as the divalent metal atom is a barium ferrite, and the hexagonal ferrite including a strontium atom is a strontium ferrite. In addition, the hexagonal ferrite may be a mixed crystal of two or more hexagonal ferrites. As an example of the mixed crystal, a mixed crystal of the barium ferrite and the strontium ferrite can be used.

As an index for a particle size of the ferromagnetic hexagonal ferrite powder, an activation volume can be used. The “activation volume” is a unit of magnetization reversal. Regarding the activation volume described in the invention and the specification, magnetic field sweep rates of a coercivity Hc measurement part at time points of 3 minutes and 30 minutes are measured by using an oscillation sample type magnetic-flux meter in an environment of an atmosphere temperature 23° C.±1° C., and the activation volume is a value acquired from the following relational expression of He and an activation volume V.

Hc=2 Ku/Ms {1−[(kT/KuV)ln(At/0.693)]^(1/2)}

[In the expression, Ku: anisotropy constant, Ms: saturation magnetization, k: Boltzmann's constant, T: absolute temperature, V: activation volume, A: spin precession frequency, and t: magnetic field reversal time]

As a method for achieving high-density recording, a method of decreasing a particle size of ferromagnetic powder included in a magnetic layer and increasing a filling percentage of the ferromagnetic powder of the magnetic layer is used. From this viewpoint, the activation volume of the ferromagnetic hexagonal ferrite powder is preferably equal to or smaller than 2,500 nm³, more preferably equal to or smaller than 2,300 nm³, and even more preferably equal to or smaller than 2,000 nm³. Meanwhile, from a viewpoint of stability of magnetization, the activation volume is, for example, preferably equal to or greater than 800 nm³, more preferably equal to or greater than 1,000 nm³, and even more preferably equal to or greater than 1,200 nm³.

The shape of the particle configuring the ferromagnetic hexagonal ferrite powder is specified by imaging the ferromagnetic hexagonal ferrite powder at a magnification ratio of 100,000 with a transmission electron microscope, and tracing an outline of a particle (primary particle) with a digitizer on a particle image obtained by printing the image on printing paper so that the total magnification of 500,000. The primary particle is an independent particle which is not aggregated. The imaging with a transmission electron microscope is performed by a direct method with a transmission electron microscope at an acceleration voltage of 300 kV. The transmission electron microscope observation and measurement can be, for example, performed with a transmission electron microscope H-9000 manufactured by Hitachi, Ltd. and image analysis software KS-400 manufactured by Carl Zeiss. Regarding the shape of the particle configuring the ferromagnetic hexagonal ferrite powder, a “planar shape” is a shape having two plate surfaces facing each other. Meanwhile, among the shapes of the particles not having such a plate surface, a shape having distinguished long axis and short axis is an “elliptical shape”. The long axis is determined as an axis (linear line) having the longest length of the particle. In contrast, the short axis is determined as an axis having the longest length of the particle in a linear line orthogonal to the long axis. A shape not having distinguished long axis and short axis, that is, a shape in which the length of the long axis is the same as the length of the short axis is a “sphere shape”. From the shapes, a shape in which the long axis and the short axis are hardly specified, is called an undefined shape. The imaging with a transmission electron microscope for specifying the shapes of the particles is performed without performing the alignment process with respect to the imaging target powder. The shape of the ferromagnetic hexagonal ferrite powder used for the preparation of the magnetic layer forming composition and the ferromagnetic hexagonal ferrite powder included in the magnetic layer may be any one of the planar shape, the elliptical shape, the sphere shape, and the undefined shape.

An average particle size of various powders disclosed in the invention and the specification is an arithmetical mean of the values obtained regarding randomly extracted 500 particles by using the particle image which is captured as described above. The average particle size shown in the examples which will be described later is a value obtained by using transmission electron microscope H-9000 manufactured by Hitachi, Ltd. as the transmission electron microscope and image analysis software KS-400 manufactured by Carl Zeiss as the image analysis software.

For details of the ferromagnetic hexagonal ferrite powder, descriptions disclosed in paragraphs 0134 to 0136 of JP2011-216149A can be referred to, for example.

The content (filling percentage) of the ferromagnetic hexagonal ferrite powder of the magnetic layer is preferably 50% to 90% by mass and more preferably 60% to 90% by mass. The components other than the ferromagnetic hexagonal ferrite powder of the magnetic layer are at least a binding agent and non-magnetic powder, and one or more kinds of additives can be randomly included. A high filling percentage of the ferromagnetic hexagonal ferrite powder of the magnetic layer is preferable, from a viewpoint of improving recording density.

Binding Agent and Curing Agent

The magnetic tape is a coating type magnetic tape, and the magnetic layer includes a binding agent. The binding agent is one or more kinds of resin. As the binding agent, various resins normally used as a binding agent of the coating type magnetic recording medium can be used. For example, as the binding agent, a resin selected from a polyurethane resin, a polyester resin, a polyamide resin, a vinyl chloride resin, an acrylic resin obtained by copolymerizing styrene, acrylonitrile, or methyl methacrylate, a cellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylal resin such as polyvinyl acetal or polyvinyl butyral can be used alone or a plurality of resins can be mixed with each other to be used. Among these, a polyurethane resin, an acrylic resin, a cellulose resin, and a vinyl chloride resin are preferable. These resins may be a homopolymer or a copolymer. These resins can be used as the binding agent even in the back coating layer and/or the non-magnetic layer which will be described later. For the binding agent described above, description disclosed in paragraphs 0028 to 0031 of JP2010-24113A can be referred to. An average molecular weight of the resin used as the binding agent can be, for example, 10,000 to 200,000 as a weight-average molecular weight. The weight-average molecular weight of the invention and the specification is a value obtained by performing polystyrene conversion of a value measured by gel permeation chromatography (GPC). As the measurement conditions, the following conditions can be used. The weight-average molecular weight shown in examples which will be described later is a value obtained by performing polystyrene conversion of a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8 mmID (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

A curing agent can also be used together with a resin which can be used as the binding agent. As the curing agent, in one aspect, a thermosetting compound which is a compound in which a curing reaction (crosslinking reaction) proceeds due to heating can be used, and in another aspect, a photocurable compound in which a curing reaction (crosslinking reaction) proceeds due to light irradiation can be used. At least a part of the curing agent is included in the magnetic layer in a state of being reacted (crosslinked) with other components such as the binding agent, by proceeding the curing reaction in the magnetic layer forming step. In a case where a composition used for forming other layers such as the back coating layer forming composition includes a curing agent, the same applies to a layer formed by using this composition. The preferred curing agent is a thermosetting compound, polyisocyanate is suitable. For details of the polyisocyanate, descriptions disclosed in paragraphs 0124 and 0125 of JP2011-216149A can be referred to, for example. The amount of the curing agent can be, for example, 0 to 80.0 parts by mass with respect to 100.0 parts by mass of the binding agent in the magnetic layer forming composition, and is preferably 50.0 to 80.0 parts by mass, from a viewpoint of improvement of strength of the magnetic layer.

Non-Magnetic Powder

Examples of non-magnetic powder include in the magnetic layer include non-magnetic powder which can function as a projection formation agent which forms projections suitably protruded from the surface of the magnetic layer (for example, referred to as a “projection formation agent”), and non-magnetic powder which can function as an abrasive (for example, referred to as an “abrasive”). The inventors have surmised that, in the magnetic layer having the XRD intensity ratio of 0.5 to 4.0, the sinking of the particles of the non-magnetic powder into the magnetic layer which can be suitably controlled, causes prevention of the occurrence of the edge damage. Details of such a surmise are as described above.

As the projection formation agent which is one aspect of the non-magnetic powder, various non-magnetic powders normally used as a projection formation agent can be used. These may be inorganic substances or organic substances. In one aspect, from a viewpoint of homogenization of friction properties, particle size distribution of the projection formation agent is not polydispersion having a plurality of peaks in the distribution and is preferably monodisperse showing a single peak. From a viewpoint of availability of monodisperse particles, the projection formation agent is preferably powder of inorganic substances (inorganic powder). Examples of the inorganic powder include powder of inorganic oxide such as metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide, and metal sulfide, and powder of inorganic oxide is preferable. The projection formation agent is more preferably colloid particles and even more preferably inorganic oxide colloid particles. In addition, from a viewpoint of availability of monodisperse particles, the inorganic oxide configuring the inorganic oxide colloid particles are preferably silicon dioxide (silica). The inorganic oxide colloid particles are more preferably colloidal silica (silica colloid particles). In the invention and the specification, the “colloid particles” are particles which are not precipitated and dispersed to generate a colloidal dispersion, in a case where 1 g of the particles is added to 100 mL of at least one organic solvent of at least methyl ethyl ketone, cyclohexanone, toluene, or ethyl acetate, or a mixed solvent including two or more kinds of the solvent described above at a random mixing ratio. In addition, in another aspect, the projection formation agent is preferably carbon black.

An average particle size of the projection formation agent is, for example, 30 to 300 nm and is preferably 40 to 200 nm.

The abrasive which is another aspect of the non-magnetic powder is preferably non-magnetic powder having Mohs hardness exceeding 8 and more preferably non-magnetic powder having Mohs hardness equal to or greater than 9. A maximum value of Mohs hardness is 10 of diamond. Specifically, powders of alumina (Al₂O₃), silicon carbide, boron carbide (B₄C), SiO₂, TiC, chromium oxide (Cr₂O₃), cerium oxide, zirconium oxide (ZrO₂), iron oxide, diamond, and the like can be used, and among these, alumina powder such as a-alumina and silicon carbide powder are preferable. In addition, regarding the particle size of the abrasive, a specific surface area which is an index for the particle size is, for example, equal to or greater than 14 m²/g, and is preferably 16 m²/g and more preferably 18 m²/g. Further, the specific surface area of the abrasive can be, for example, equal to or smaller than 40 m²/g. The specific surface area is a value obtained by a nitrogen adsorption method (also referred to as a Brunauer-Emmett-Teller (BET) 1 point method), and is a value measured regarding primary particles. Hereinafter, the specific surface area obtained by such a method is also referred to as a BET specific surface area.

In addition, from a viewpoint that the projection formation agent and the abrasive can exhibit the functions thereof in more excellent manner, the content of the projection formation agent in the magnetic layer is preferably 1.0 to 4.0 parts by mass and more preferably 1.5 to 3.5 parts by mass with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder. Meanwhile, the content of the abrasive in the magnetic layer is preferably 1.0 to 20.0 parts by mass, more preferably 3.0 to 15.0 parts by mass, and even more preferably 4.0 to 10.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic hexagonal ferrite powder.

Other Components

The magnetic layer may include one or more kinds of additives, if necessary, together with the various components described above. As the additives, a commercially available product can be suitably selected and used according to the desired properties. Alternatively, a compound synthesized by a well-known method can be used as the additives. As the additives, the curing agent described above is used as an example. In addition, examples of the additive which can be included in the magnetic layer include a lubricant, a dispersing agent, a dispersing assistant, an antibacterial agent, an antistatic agent, and an antioxidant. For example, as the dispersing agent, a well-known dispersing agent such as a carboxy group-containing compound or a nitrogen-containing compound can be used. For example, the nitrogen-containing compound may be any of a primary amine represented by NH₂R, a secondary amine represented by NHR₂, and a tertiary amine represented by NR₃. In the above description, R represents a random structure configuring the nitrogen-containing compound, and a plurality of Rs may be the same as each other or different from each other. The nitrogen-containing compound may be a compound (polymer) having a plurality of repeating structure in a molecule. It is thought that a nitrogen-containing part of the nitrogen-containing compound which functions as an adsorption part to the surface of the particle of the ferromagnetic hexagonal ferrite powder is a reason why the nitrogen-containing compound can function as the dispersing agent. As the carboxy group-containing compound, fatty acid such as oleic acid can be used, for example. It is thought that a carboxy group which functions as an adsorption part to the surface of the particle of the ferromagnetic hexagonal ferrite powder is a reason why the carboxy group-containing compound can function as the dispersing agent. It is also preferable to use the carboxy group-containing compound and the nitrogen-containing compound in combination.

As an example of the additive which can be used in the magnetic layer including the abrasive, a dispersing agent disclosed in paragraphs 0012 to 0022 of JP2013-131285A can be used as a dispersing agent for improving dispersibility of the abrasive of the magnetic layer forming composition. It is preferable to improve dispersibility of the non-magnetic powder such as the abrasive in the magnetic layer forming composition, in order to decrease the magnetic layer surface roughness Ra.

Back Coating Layer

Fatty Acid Ester

The magnetic tape includes fatty acid ester in the back coating layer. The surmise of the inventors regarding the fatty acid ester, the difference (S_(after)−S_(before)), the FWHM_(before), and the FWHM_(after) is as described above.

The fatty acid ester may be included in the back coating layer alone as one type or two or more types thereof may be included therein. Examples of fatty acid ester include esters of lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, and elaidic acid. Specific examples thereof include butyl myristate, butyl palmitate, butyl stearate (butyl stearate), neopentyl glycol dioleate, sorbitan monostearate, sorbitan distearate, sorbitan tristearate, oleyl oleate, isocetyl stearate, isotridecyl stearate, octyl stearate, isooctyl stearate, amyl stearate, and butoxyethyl stearate.

The content of fatty acid ester in the back coating layer is, for example, 0.1 to 10.0 parts by mass and is preferably 1.0 to 5.0 parts by mass with respect to 100.0 parts by mass of non-magnetic powder included in the back coating layer. In a case of using two or more kinds of different fatty acid esters as the fatty acid ester, the content thereof is the total content thereof. In the invention and the specification, a given component may be used alone or used in combination of two or more kinds thereof, unless otherwise noted. In a case of using two or more kinds of the given components, the content of this component is the total content of the two or more kinds thereof, unless otherwise noted.

Other Lubricants

The magnetic tape includes fatty acid ester which is one kind of lubricants in the back coating layer, and may or may not include lubricants other than fatty acid ester in the back coating layer. As the lubricant other than fatty acid ester, fatty acid or fatty acid amide can be used. The fatty acid and fatty acid amide are generally known as a component which can function as a boundary lubricant.

Examples of fatty acid include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, behenic acid, erucic acid, and elaidic acid, and stearic acid, myristic acid, and palmitic acid are preferable, and stearic acid is more preferable. Fatty acid may be included in the back coating layer in a state of salt such as metal salt.

As fatty acid amide, amide of various fatty acid described above is used, and examples thereof include lauric acid amide, myristic acid amide, palmitic acid amide, and stearic acid amide.

In a case of using fatty acid and a derivative of fatty acid (ester and amide) in combination, a part derived from fatty acid of the fatty acid derivative preferably has a structure which is the same as or similar to that of fatty acid used in combination. As an example, in a case of using stearic acid as fatty acid, it is preferable to use stearic acid ester and/or stearic acid amide.

The content of fatty acid in the back coating layer is, for example, 0 to 10.0 parts by mass, preferably 0.1 to 10.0 parts by mass, and more preferably 1.0 to 7.0 parts by mass, with respect to 100.0 parts by mass of the non-magnetic powder included in the back coating layer. The content of fatty acid amide in the back coating layer is, for example, 0 to 3.0 parts by mass, preferably 0 to 2.0 parts by mass, and more preferably 0 to 1.0 part by mass with respect to 100.0 parts by mass of the non-magnetic powder included in the back coating layer.

The various lubricants described above may be included in the magnetic layer and/or the non-magnetic layer which is randomly provided. Regarding the kind and/or the content of the lubricant in each layer, a well-known technology regarding the list of each layer can be used.

Non-Magnetic Powder

As non-magnetic powder including in the back coating layer, any one or both of carbon black and non-magnetic powder other than the carbon black can be used. As the non-magnetic powder other than the carbon black, inorganic powder can be used. Examples of the inorganic powder include inorganic powder of iron oxide such as a-iron oxide, titanium oxide such as titanium dioxide, cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO₂, SiO₂, Cr₂O₃, α-alumina, β-alumina, γ-alumina, goethite, corundum, silicon nitride, titanium carbide, magnesium oxide, boron nitride, molybdenum disulfide, copper oxide, MgCO₃, CaCO₃, BaCO₃, SrCO₃, BaSO₄, and silicon carbide. The preferable inorganic powder is inorganic oxide powder, more preferably a-iron oxide and titanium oxide, and even more preferably a-iron oxide. For the non-magnetic powder included in the back coating layer, the description which will be described later regarding the non-magnetic powder included in the non-magnetic layer can be referred to.

Generally, the inorganic powder tends to have excellent dispersibility in the back coating layer forming composition, compared to that of carbon black. An increase in dispersibility of the non-magnetic powder in the back coating layer forming composition can contribute to a decrease in variation of the surface shape of the back coating layer. Accordingly, as a method of adjusting the FWHM_(after) measured after the vacuum heating, which is considered to become a small value, as the variation in the surface shape of the back coating layer decreases, a method of adjusting the kind of the non-magnetic powder included in the back coating layer, and a mixing ratio, in a case of including two or more kinds of the non-magnetic powders can be used. For example, as main powder of the non-magnetic powder of the back coating layer (non-magnetic powder, the largest amount of which is included based on mass, among the non-magnetic powder), the inorganic powder is preferably used. In a case where the non-magnetic powder included in the back coating layer is one or more kinds of the non-magnetic powder selected from the group consisting of the inorganic powder and carbon black, the content of the inorganic powder with respect to 100.0 parts by mass of a total amount of the non-magnetic powder is preferably greater than 50.0 parts by mass and equal to or smaller than 100.0 parts by mass, more preferably 60.0 parts by mass to 100.0 parts by mass, even more preferably 70.0 parts by mass to 100.0 parts by mass, and still more preferably 80.0 parts by mass to 100.0 parts by mass. In addition, for the content (filling percentage) of the non-magnetic powder in the back coating layer, the description which will be described later regarding the non-magnetic powder of the non-magnetic layer can be referred to.

An average particle size of the non-magnetic powder can be, for example, 10 to 200 nm. An average particle size of the inorganic powder is preferably 50 to 200 nm and more preferably 80 to 150 nm. Meanwhile, an average particle size of the carbon black is preferably 10 to 50 nm and more preferably 15 to 30 nm.

In addition, the dispersibility of the non-magnetic powder in the back coating layer forming composition can be increased by using a well-known dispersing agent, reinforcing dispersion conditions, and the like.

A preferable aspect of a method of adjusting FWHM_(before) measured before the vacuum heating will be described later.

Other Components

The back coating layer further includes a binding agent and can randomly include well-known additives. For other details of the binding agent and additives of the back coating layer, a well-known technology regarding the back coating layer can be applied, and a well-known technology regarding the magnetic layer and/or the non-magnetic layer can also be applied.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic tape may include a magnetic layer directly on a surface of a non-magnetic support, or may include a magnetic layer through the non-magnetic layer including non-magnetic powder and a binding agent on the surface of the non-magnetic support. The non-magnetic powder used in the non-magnetic layer may be inorganic powder or organic powder. In addition, carbon black and the like can be used. Examples of the inorganic powder include powders of metal, metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide, and metal sulfide. These non-magnetic powder can be purchased as a commercially available product or can be manufactured by a well-known method. For details thereof, descriptions disclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referred to. For carbon black which can be used in the non-magnetic layer, descriptions disclosed in paragraphs 0040 and 0041 of JP2010-24113A can be referred to. The content (filling percentage) of the non-magnetic powder of the non-magnetic layer is preferably 50% to 90% by mass and more preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of the non-magnetic layer, the well-known technology regarding the non-magnetic layer can be applied. In addition, in regards to the type and the content of the binding agent, and the type and the content of the additive, for example, the well-known technology regarding the magnetic layer can be applied.

The non-magnetic layer of the magnetic tape also includes a substantially non-magnetic layer including a small amount of ferromagnetic powder as impurities or intentionally, together with the non-magnetic powder. Here, the substantially non-magnetic layer is a layer having a residual magnetic flux density equal to or smaller than 10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m(100 Oe), or a layer having a residual magnetic flux density equal to or smaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have a residual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support (hereinafter, also simply referred to as a “support”) will be described. As the non-magnetic support, well-known components such as polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamide imide, aromatic polyamide subjected to biaxial stretching are used. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable. Corona discharge, plasma treatment, easy-bonding treatment, or heating treatment may be performed with respect to these supports in advance.

Various Thickness

The thickness of the non-magnetic support is preferably 3.00 to 20.00 μm, more preferably 3.00 to 10.00 μm, even more preferably 3.00 to 6.00 μm, and particularly preferably 3.00 to 4.50 μm.

The thickness of the magnetic layer is preferably equal to or smaller than 0.15 μm and more preferably equal to or smaller than 0.10 μm, from a viewpoint of realizing recording at high density which is recently required. The thickness of the magnetic layer is even more preferably 0.01 to 0.10 μm. The magnetic layer may be at least single layer, the magnetic layer may be separated into two or more layers having different magnetic properties, and a configuration of a well-known multilayered magnetic layer can be applied. A thickness of the magnetic layer in a case where the magnetic layer is separated into two or more layers is the total thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μm and preferably 0.10 to 1.00 μm.

A thickness of the back coating layer is preferably equal to or smaller than 0.90 μm, more preferably 0.10 to 0.70 μm, even more preferably 0.10 to 0.55 μm, and still more preferably 0.10 to 0.50 μm.

The thicknesses of various layers of the magnetic tape and the non-magnetic support can be acquired by a well-known film thickness measurement method. As an example, a cross section of the magnetic tape in a thickness direction is, for example, exposed by a well-known method of ion beams or microtome, and the exposed cross section is observed with a scanning electron microscope. In the cross section observation, various thicknesses can be acquired as a thickness acquired at one position of the cross section in the thickness direction, or an arithmetical mean of thicknesses acquired at a plurality of positions of two or more positions, for example, two positions which are randomly extracted. In addition, the thickness of each layer may be acquired as a designed thickness calculated according to the manufacturing conditions.

Manufacturing Method

Preparation of Each Layer Forming Composition

Each composition for forming the magnetic layer, the back coating layer, or the non-magnetic layer, normally includes a solvent, together with various components described above. As the solvent, various organic solvents generally used for manufacturing a coating type magnetic recording medium can be used. Among those, from a viewpoint of solubility of the binding agent normally used in the coating type magnetic recording medium, each layer forming composition preferably includes one or more ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, and tetrahydrofuran. The amount of the solvent of each layer forming composition is not particularly limited, and can be set to be the same as that of each layer forming composition of a typical coating type magnetic recording medium. In addition, steps of preparing a composition for forming each layer can generally include at least a kneading step, a dispersing step, and a mixing step provided before and after these steps, if necessary. Each step may be divided into two or more stages. All of raw materials used in the invention may be added at an initial stage or in a middle stage of each step. In addition, each raw material may be separately added in two or more steps. For example, a binding agent may be separately added in a kneading step, a dispersing step, and a mixing step for adjusting viscosity after the dispersion. In the manufacturing step of the magnetic tape, a well-known manufacturing technology of the related art can be used in a part or all of the steps. In the kneading step, an open kneader, a continuous kneader, a pressure kneader, or a kneader having a strong kneading force such as an extruder is preferably used. The details of the kneading processes of these kneaders are disclosed in JP1989-106338A (JP-H01-106338A) and JP1989-79274A (JP-H01-79274A). As a dispersing machine, a well-known dispersing machine can be used. The each layer forming composition may be filtered by a well-known method before performing the coating step. The filtering can be performed by using a filter, for example. As the filter used in the filtering, a filter having a hole diameter of 0.01 to 3 μm (for example, filter made of glass fiber or filter made of polypropylene) can be used, for example.

Regarding the dispersion process of the magnetic layer forming composition, it is preferable to prevent the occurrence of chipping as described above. In order to realize the prevention, it is preferable to perform the dispersion process of the ferromagnetic hexagonal ferrite powder by a dispersion process having two stages, in which a coarse aggregate of the ferromagnetic hexagonal ferrite powder is crushed by the dispersion process in a first stage, and the dispersion process in a second stage, in which a collision energy applied to particles of the ferromagnetic hexagonal ferrite powder due to collision with the dispersion beads is smaller than that in the first dispersion process, is performed, in the step of preparing the magnetic layer forming composition. According to such a dispersion process, it is possible to improve dispersibility of the ferromagnetic hexagonal ferrite powder and prevent the occurrence of chipping.

As a preferred aspect of the dispersion process having two stages, a dispersion process including a first stage of obtaining a dispersion liquid by performing the dispersion process of the ferromagnetic hexagonal ferrite powder, the binding agent, and the solvent under the presence of first dispersion beads, and a second stage of performing the dispersion process of the dispersion liquid obtained in the first stage under the presence of second dispersion beads having smaller bead diameter and density than those of the first dispersion beads can be used. Hereinafter, the dispersion process of the preferred aspect described above will be further described.

In order to increase the dispersibility of the ferromagnetic hexagonal ferrite powder, the first stage and the second stage are preferably performed as the dispersion process before mixing the ferromagnetic hexagonal ferrite powder and other powder components. For example, in a case of forming the magnetic layer including the non-magnetic powder, the first stage and the second stage are preferably performed as a dispersion process of a solution (magnetic liquid) including ferromagnetic hexagonal ferrite powder, a binding agent, a solvent, and randomly added additives, before mixing the non-magnetic powder.

A bead diameter of the second dispersion bead is preferably equal to or smaller than 1/100 and more preferably equal to or smaller than 1/500 of a bead diameter of the first dispersion bead. The bead diameter of the second dispersion bead can be, for example, equal to or greater than 1/10,000 of the bead diameter of the first dispersion bead. However, there is no limitation to this range. The bead diameter of the second dispersion bead is, for example, preferably 80 to 1,000 nm. Meanwhile, the bead diameter of the first dispersion bead can be, for example, 0.2 to 1.0 mm.

The bead diameter of the invention and the specification is a value measured by the same method as the measurement method of the average particle size of the powder described above.

The second stage is preferably performed under the conditions in which the amount of the second dispersion beads is equal to or greater than 10 times of the amount of the ferromagnetic hexagonal ferrite powder, and is more preferably performed under the conditions in which the amount of the second dispersion beads is 10 times to 30 times of the amount of the ferromagnetic hexagonal ferrite powder, based on mass.

Meanwhile, the amount of the dispersion beads in the first stage is preferably in the range described above.

The second dispersion beads are beads having lower density than that of the first dispersion beads. The “density” is obtained by dividing the mass (unit: g) of the dispersion beads by volume (unit: cm³). The measurement is performed by the Archimedes method. The density of the second dispersion beads is preferably equal to or lower than 3.7 g/cm³ and more preferably equal to or lower than 3.5 g/cm³. The density of the second dispersion beads may be, for example, equal to or higher than 2 0 g/cm³ or may be lower than 2.0 g/cm³. As the preferred second dispersion beads from a viewpoint of density, diamond beads, silicon carbide beads, or silicon nitride beads can be used, and as preferred second dispersion beads from a viewpoint of density and hardness, diamond beads can be used.

Meanwhile, as the first dispersion beads, dispersion beads having density exceeding 3.7 g/cm³ are preferable, dispersion beads having density equal to or higher than 3.8 g/cm³ are more preferable, and dispersion beads having density equal to or higher than 4.0 g/cm³ are even more preferable. The density of the first dispersion beads may be, for example, equal to or smaller than 7.0 g/cm³ or may exceed 7.0 g/cm³. As the first dispersion beads, zirconia beads or alumina beads are preferably used, and zirconia beads are more preferably used.

The dispersion time is not particularly limited and may be set in accordance with the kind of a dispersing machine used.

Coating Step

The magnetic layer can be formed by directly applying the magnetic layer forming composition onto the surface of the non-magnetic support or by performing multilayer coating of the magnetic layer forming composition with the non-magnetic layer forming composition in order or at the same time. The back coating layer can be formed by applying the back coating layer forming composition to a surface side of the non-magnetic support opposite to a side provided with the magnetic layer (or to be provided with the magnetic layer). For details of the coating for forming each layer, a description disclosed in paragraph 0066 of JP2010-231843A can be referred to.

Other Steps

For details of various other steps for manufacturing the magnetic tape, descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A can be referred to. For example, preferably, while the coating layer of the magnetic layer forming composition is wet, an alignment process of the ferromagnetic hexagonal ferrite powder in the coating layer is performed in an alignment zone. For the alignment process, various well-known technologies such as a description disclosed in a paragraph 0067 of JP2010-231843A can be used. As described above, it is preferable to perform the homeotropic alignment process as the alignment process, from a viewpoint of controlling the XRD intensity ratio. Regarding the alignment process, the above description can also be referred to. In addition, it is also preferable that the surface smoothing treatment is performed. The smoothness of the surface of the magnetic layer of the magnetic tape can be increased by the surface smoothing treatment. The surface smoothing treatment is preferably performed by a calender process. For details of the calender process, for example, description disclosed in a paragraph 0026 of JP2010-231843A can be referred to. As the calender process is reinforced, the surface of the magnetic tape can be smoothened (that is, the value of the magnetic layer surface roughness Ra can be decreased). The calender process is reinforced, as the surface temperature (calender temperature) of a calender roll is increased and/or as calender pressure is increased.

One Aspect of Preferred Manufacturing Method

As a preferred manufacturing method, a manufacturing method of applying vibration to the back coating layer in any stage after applying the back coating layer forming composition can be used, in order to improve uniformity of the thickness of the liquid film of fatty acid ester on the surface of the back coating layer of the magnetic tape. The vibration application can be performed in any stage that is after drying or curing the coating layer of the back coating layer forming composition. It is thought that, by adding vibration, the liquid film of fatty acid ester on the surface of the coating layer flows and the uniformity of the thickness of the liquid film of fatty acid ester on the surface of the back coating layer to be formed is improved. It is surmised that it is preferable to improve uniformity of the thickness of the liquid film, in order to control the FWHM_(before) to be greater than 0 nm and equal to or smaller than 10.0 nm. That is, in one aspect of the preferable manufacturing method, a back coating layer forming step including, applying the back coating layer forming composition including non-magnetic powder, a binding agent, and fatty acid ester on one surface side of the non-magnetic support and drying to form a coating layer, and applying vibration to the formed coating layer coating layer to form the back coating layer. The manufacturing step is identical to the typical manufacturing step of the magnetic tape, except for applying vibration to the coating layer of the back coating layer forming composition, and the details thereof are as described above.

Means for applying vibration are not particularly limited. For example, the vibration can be applied to the back coating layer by bringing the surface of the non-magnetic support, to which the back coating layer forming composition is applied, on a side opposite to the back coating layer, into contact with a vibration imparting unit. The non-magnetic support, provided with the back coating layer formed, may run while coming into contact with a vibration imparting unit. The vibration imparting unit, for example, includes an ultrasonic vibrator therein, and accordingly, vibration can be applied to a product coming into contact with the unit. It is possible to adjust the vibration applied to the back coating layer by a vibration frequency and strength of the ultrasonic vibrator, and/or the contact time with the vibration imparting unit. For example, the contact time can be adjusted by a running speed of the non-magnetic support, provided with the back coating layer formed, while coming into contact with the vibration imparting unit. The vibration imparting conditions are not particularly limited, and may be set so as to control the full width at half maximum of the spacing distribution described above, particularly, the FWHM_(before) measured before vacuum heating. In order to set the vibration imparting conditions, a preliminary experiment can be performed before the actual manufacturing, and the conditions can be optimized.

As described above, it is possible to obtain a magnetic tape included in the magnetic tape device according to one aspect of the invention. However, the manufacturing method described above is merely an example, the magnetic layer surface roughness Ra, the XRD intensity ratio, the vertical squareness ratio, the FWHM_(before) and the FWHM_(after) of the back coating layer, and the difference (S_(after)−S_(before)) can be controlled to be in respective ranges described above by a random method capable of adjusting the magnetic layer surface roughness Ra, the XRD intensity ratio, the vertical squareness ratio, and the logarithmic decrement, and such an aspect is also included in the invention. The magnetic tape is generally accommodated in a magnetic tape cartridge and the magnetic tape cartridge is mounted in the magnetic tape device (drive). A servo pattern can also be formed in the magnetic tape by a well-known method, in order to allow head tracking servo to be performed in the drive. The drive includes at least the magnetic tape mounted on the magnetic tape cartridge, and one or more magnetic heads for recording and/or reproducing information. Even in a case where the magnetic tape is allowed to run in the drive and feeding and winding of the magnetic tape from a reel of the magnetic tape cartridge are repeated, it is possible to prevent the occurrence of the edge damage, according to the magnetic tape according to one aspect of the invention. In addition, as a running speed of the magnetic tape in the drive increases, the recording of information and reproducing of the recorded information can be performed for a short time. Here, the running speed of the magnetic tape is a relative speed of the magnetic tape and the magnetic head in the drive and is normally set in a controller of the drive. As the running speed of the magnetic tape increases, the contact state between the surface of the magnetic layer and the surface of the back coating layer tends to be unstable, and accordingly, the edge damage easily occurs. Even in such a case, it is possible to prevent the occurrence of the edge damage, according to the magnetic tape according to one aspect of the invention. The magnetic tape according to one aspect of the invention is suitably used in a drive in which the running speed of the magnetic tape is equal to or higher than 3 m/sec (for example, 3 to 20 m/sec).

EXAMPLES

Hereinafter, the invention will be described with reference to examples. However, the invention is not limited to aspects shown in the examples. “Parts” and “%” in the following description mean “parts by mass” and “% by mass”, unless otherwise noted. In addition, steps and evaluations described below are performed in an environment of an atmosphere temperature of 23° C.±1° C., unless otherwise noted.

1. Manufacturing of Magnetic Tape

Example 1

A list of each layer forming composition is shown below.

List of Magnetic Layer Forming Composition

Magnetic Liquid

Plate-shaped ferromagnetic hexagonal ferrite powder (M-type barium ferrite): 100.0 parts

-   -   (Activation volume: 1,500 nm³)

Oleic acid: 2.0 parts

A vinyl chloride copolymer (MR-104 manufactured by Zeon Corporation): 10.0 parts SO₃Na group-containing polyurethane resin: 4.0 parts

-   -   (Weight-average molecular weight: 70,000, SO₃Na group: 0.07         meq/g)

An amine-based polymer (DISPERBYK-102 manufactured by BYK Additives & Instruments): 6.0 parts

Methyl ethyl ketone: 150.0 parts

Cyclohexanone: 150.0 parts

Abrasive Solution

α-alumina: 6.0 parts

-   -   (BET specific surface area: 19 m²/g, Mohs hardness: 9)

SO₃Na group-containing polyurethane resin: 0.6 parts

-   -   (Weight-average molecular weight: 70,000, SO₃Na group: 0.1         meq/g)

2,3-Dihydroxynaphthalene: 0.6 parts

Cyclohexanone: 23.0 parts

Projection Formation Agent Liquid

Colloidal silica: 2.0 parts

-   -   (Average particle size: 80 nm)

Methyl ethyl ketone: 8.0 parts

Lubricant and Curing Agent Liquid

Stearic acid: 3.0 parts

Stearic acid amide: 0.3 parts

Butyl stearate: 6.0 parts

Methyl ethyl ketone: 110.0 parts

Cyclohexanone: 110.0 parts

Polyisocyanate (CORONATE (registered trademark) L manufactured by Tosoh Corporation): 3.0 parts

List of Non-Magnetic Layer Forming Composition

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

-   -   (Average particle size: 10 nm, BET specific surface area: 75         m²/g)

Carbon black: 25.0 parts

-   -   (Average particle size: 20 nm)

A SO₃Na group-containing polyurethane resin: 18.0 parts

-   -   (Weight-average molecular weight: 70,000, content of SO₃Na         group: 0.2 meq/g)

Stearic acid: 1.0 parts

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

List of Back Coating Layer Forming Composition

Non-magnetic powder: 100.0 parts

-   -   inorganic powder (α-iron oxide): see Table 1 for ratio     -   (Average particle size: 150 nm, Average acicular ratio: 7, BET         specific surface area: 52 m²/g)     -   Carbon black: see Table 1 for ratio     -   (Average particle size: 20 nm)

A vinyl chloride copolymer: 13.0 parts

A sulfonic acid salt group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Cyclohexanone: 140.0 parts

Methyl ethyl ketone: 170.0 parts

Butyl stearate: see Table 1

Stearic acid: 2.0 parts

Polyisocyanate (CORONATE L manufactured by Tosoh Corporation): 5.0 parts

Methyl ethyl ketone: 400.0 parts

Preparation of Magnetic Layer Forming Composition

The magnetic layer forming composition was prepared by the following method.

A dispersion liquid A was prepared by dispersing (first stage) various components of the magnetic liquid with a batch type vertical sand mill by using zirconia beads having a bead diameter of 0.5 mm (first dispersion beads, density of 6.0 g/cm³) for 24 hours, and then performing filtering with a filter having a hole diameter of 0.5 μm. The used amount of zirconia beads was 10 times of the amount of the ferromagnetic hexagonal barium ferrite powder based on mass.

After that, a dispersion liquid (dispersion liquid B) was prepared by dispersing (second stage) dispersion liquid A with a batch type vertical sand mill by using diamond beads having a bead diameter shown in Table 1 (second dispersion beads, density of 3.5 g/cm³) for 1 hour, and then separating diamond beads by using a centrifugal separator. The magnetic liquid is the dispersion liquid B obtained as described above. The used amount of diamond beads was 10 times of the amount of the ferromagnetic hexagonal ferrite barium powder based on mass.

Regarding the abrasive solution, various components of the abrasive solution were mixed with each other and put in a transverse beads mill disperser together with zirconia beads having a bead diameter of 0.3 mm, so as to perform the adjustment so that a value of bead volume/(abrasive solution volume+bead volume) was 80%, the beads mill dispersion process was performed for 120 minutes, the liquid after the process was extracted, and an ultrasonic dispersion filtering process was performed by using a flow type ultrasonic dispersion filtering device. By doing so, the abrasive solution was prepared.

The magnetic layer forming composition was prepared by introducing the prepared magnetic liquid, the abrasive solution, the projection formation agent solution, and the lubricant and curing agent solution in a dissolver, stirring the mixture at a circumferential speed of 10 m/sec for 30 minutes, and performing a process of 3 passes at a flow rate of 7.5 kg/min with a flow type ultrasonic disperser stirring device, and filtering the mixture with a filter having a hole diameter of 1 μm.

The activation volume of the ferromagnetic hexagonal ferrite powder described above is a value calculated by performing measurement by using powder which is the same powder lot as the ferromagnetic hexagonal ferrite powder used in the preparation of the magnetic layer forming composition. The magnetic field sweep rates in the coercivity He measurement part at timing points of 3 minutes and 30 minutes were measured by using an oscillation sample type magnetic-flux meter (manufactured by Toei Industry Co., Ltd.), and the activation volume was calculated from the relational expression described above. The measurement was performed in the environment of 23° C.±1° C.

Preparation of Non-Magnetic Layer Forming Composition

A non-magnetic layer forming composition was prepared by dispersing various components of the non-magnetic layer forming composition with a batch type vertical sand mill by using zirconia beads having a bead diameter of 0.1 mm for 24 hours, and then performing filtering with a filter having a hole diameter of 0.5 μm.

Preparation of Back Coating Layer Forming Composition

Components among various components of the back coating layer forming composition except a lubricant (butyl stearate and stearic acid), polyisocyanate, and methyl ethyl ketone (400.0 parts) were kneaded and diluted by an open kneader, and subjected to a dispersion process with a transverse beads mill disperser. After that, the lubricant (butyl stearate and stearic acid), polyisocyanate, and methyl ethyl ketone (400.0 parts) were added, stirred and mixed with a dissolver stirrer, and a back coating layer forming composition was prepared.

Manufacturing Method of Magnetic Tape

The back coating layer forming composition was applied onto one surface of a polyethylene naphthalate support having a thickness of 4.50 μm so that the thickness after the drying becomes 0.50 μm, and dried, and a coating layer was formed. Then, the support, provided with the coating layer formed, was installed in a vibration imparting device shown in FIG. 1 so that the surface thereof on a side opposite to the surface where the coating layer is formed comes into contact with the vibration imparting unit, and the support (in FIG. 1, reference numeral 101), provided with the coating layer formed, was transported at a transportation speed of 0.5 m/sec, to apply vibration to the coating layer. In FIG. 1, a reference numeral 102 denotes a guide roller (a reference numeral 102 denotes one of two guide rollers), a reference numeral 103 denotes the vibration imparting device (vibration imparting unit including the ultrasonic vibrator), and an arrow denotes a transportation direction. The time from the start of the contact of the arbitrary portion of the support, provided with the coating layer formed, with the vibration imparting unit until the end of the contact is shown in Table 1 as the vibration imparting time. The vibration imparting unit used includes an ultrasonic vibrator therein. The vibration was imparted by setting a vibration frequency and the strength of the ultrasonic vibrator as values shown in Table 1.

After that, the non-magnetic layer forming composition was applied onto the surface of the support provided with the back coating layer formed, on a side opposite to the surface where the back coating layer is formed, so that the thickness after the drying becomes 1.00 μm, and dried, and a non-magnetic layer was formed.

The magnetic layer forming composition was applied onto the surface of the formed non-magnetic layer so that the thickness after the drying becomes 0.10 μm, and a coating layer was formed. A homeotropic alignment process was performed in the alignment zone by applying a magnetic field having strength shown in Table 1 in a vertical direction with respect to the surface of the coating layer, while the formed coating layer is wet. Then, the coating layer was dried and a magnetic layer was formed.

A calender process (surface smoothing treatment) of the magnetic tape obtained as described above was performed with a calender configured of only a metal roll, at a speed of 100 m/min, linear pressure of 300 kg/cm (294 kN/m), by using a calender roll having a surface temperature shown in Table 1, and heat treatment was performed in an environment of an atmosphere temperature of 70° C. for 36 hours. After the heat treatment, the slitting was performed so as to have a width of ½ inches (0.0127 meters), and a servo pattern was formed on the magnetic layer by a commercially available servo writer.

By doing so, the magnetic tape of Example 1 was obtained.

Examples 2 to 11, Comparative Examples 1 to 11, and Reference Examples 1 and 2

A magnetic tape was manufactured in the same manner as in Example 1, except that various items shown in Table 1 were changed as shown in Table 1.

The vibration imparting time was adjusted by changing the transportation speed of the support formed with the back coating layer.

A ratio of the inorganic powder/carbon black in the back coating layer shown in Table 1 is a content of each powder based on mass with respect to 100.0 parts by mass of a total content of the non-magnetic powder (inorganic powder and carbon black).

In Table 1, in the comparative examples and the reference examples in which “none” is shown in a column of the dispersion beads and a column of the time, the magnetic layer forming composition was prepared without performing the second stage in the magnetic liquid dispersion process.

In Table 1, in the comparative examples and the reference examples in which “none” is shown in a column of the homeotropic alignment process magnetic field strength, the magnetic layer was formed without performing the alignment process.

In Table 1, in the comparative examples and the reference examples in which “not performed” is disclosed in a column of the ultrasonic vibration imparting conditions, a magnetic tape was manufactured by a manufacturing step not performing the ultrasonic vibration imparting.

2. Various Evaluations

(1) XRD Intensity Ratio

A tape sample was cut out from the manufactured magnetic tape.

Regarding the cut-out tape sample, the surface of the magnetic layer was irradiated with X-ray by using a thin film X-ray diffraction device (Smart Lab manufactured by Rigaku Corporation), and the In-Plane XRD was performed by the method described above.

The peak intensity Int(114) of the diffraction peak of the (114) plane and the peak intensity Int(110) of the diffraction peak of a (110) plane of a hexagonal ferrite crystal structure were obtained from the X-ray diffraction spectra obtained by the In-Plane XRD, and the XRD intensity ratio (Int(110)/Int(114)) was calculated.

(2) Vertical Squareness Ratio

A vertical squareness ratio of the manufactured magnetic tape was obtained by the method described above using an oscillation sample type magnetic-flux meter (manufactured by Toei Industry Co., Ltd.).

(3) Center Line Average Surface Roughness Ra Measured Regarding Surface of Magnetic Layer

The measurement regarding a measurement area of 40 μm×40 μm in the surface of the magnetic layer of the magnetic tape was performed with an atomic force microscope (AFM, Nanoscope 4 manufactured by Veeco Instruments, Inc.) in a tapping mode, and a center line average surface roughness Ra was acquired. RTESP-300 manufactured by BRUKER is used as a probe, a scan speed (probe movement speed) was set as 40 μm/sec, and a resolution was set as 512 pixel×512 pixel.

(4) FWHM_(before) and FWHM_(after) of Back Coating Layer

The FWHM_(before) and FWHM_(after) were acquired by the following method by using a tape spacing analyzer (TSA) (manufactured by Micro Physics, Inc.).

In a state where a glass sheet included in the TSA was disposed on the surface of the back coating layer of the magnetic tape, a hemisphere was pressed against the surface of the magnetic layer of the magnetic tape at a pressure of 5.05×10⁴ N/m (0.5 atm) by using a hemisphere made of urethane included in the TSA as a pressing member. In this state, a given region (150,000 to 200,000 μm²) of the surface of the back coating layer of the magnetic tape was irradiated with white light from a stroboscope included in the TSA through the glass sheet, and the obtained reflected light was received by a charge-coupled device (CCD) through an interference filter (filter selectively passing light at a wavelength of 633 nm), and thus, an interference fringe image generated on the uneven part of the region was obtained.

This image was divided into 300,000 points, a distance (spacing) between the surface of the glass sheet of each point on the magnetic tape side and the surface of the back coating layer of the magnetic tape was acquired, this was shown as a histogram, and the FWHM_(before) before vacuum heating and FWHM_(after) after vacuum heating were acquired by setting the full width at half maximum, in a case where the histogram was fit with Gaussian distribution, as the full width at half maximum of the spacing distribution.

The vacuum heating was performed by storing the magnetic tape in a vacuum constant temperature drying machine with a degree of vacuum of 200 Pa to 0.01 Mpa and at inner atmosphere temperature of 70° C. to 90° C. for 24 hours.

(5) Difference (S_(after)−S_(before)) of Back Coating Layer

The difference (S_(after)−S_(before)) was a value obtained by subtracting a mode of the histogram before the vacuum heating from a mode of the histogram after the vacuum heating obtained in the section (4).

(6) Evaluation of Edge Damage

A magnetic tape cartridge accommodating each magnetic tape (magnetic tape total length of 500 m) of the examples, the comparative examples, and the reference examples was set in a drive of Linear Tape-Open Generation 7 (LTO-G7) manufactured by IBM, and the magnetic tape was subjected to reciprocating running 1,500 times at tension of 0.6 N and a running speed of 5 m/sec, while bringing the surface of the magnetic layer into contact with the magnetic head for sliding.

The magnetic tape cartridge after the miming was set in a reference drive (LTO-G7 drive manufactured by IBM), and the magnetic tape is allowed to run to perform the recording and reproducing. A reproduction signal during the running was introduced to an external analog/digital (AD) conversion device. A signal having a reproducing signal amplitude which is decreased 70% or more than an average (average of measured values at each track) respectively in a track closest to one edge of the magnetic tape and a track closest to the other edge thereof was set as a missing pulse, a generation frequency (number of times of the generation) thereof was divided by the total length of the magnetic tape to obtain a missing pulse generation frequency per unit length of the magnetic tape (per 1 m) (unit: times/m).

As the edge damage heavily occurs, the missing pulse generation frequency obtained by the method described above increases. Accordingly, the missing pulse generation frequency obtained by the method described above becomes an index for the edge damage. In a case where the missing pulse generation frequency is equal to or smaller than 10.0 times/m, it is possible to determine that the occurrence of the edge damage is prevented to a sufficient practical level. The position where the edge damage occurs is not constant, and therefore, in this evaluation, the measurement result having a large number of missing pulses was used as the missing pulse generation frequency, among the measurement result in a track closest to one edge and the measurement result in a track closest to the other edge, and was shown in Table 1.

The results described above are shown in Table 1.

TABLE 1 Magnetic liquid dispersion process second stage Dispersion beads Used amount (mass Amount of butyl Ultrasonic vibration of beads with Homeotropic stearate in back imparting conditions respect to mass of alignment Surface coating layer Vibration ferromagnetic process temperature forming imparting Vibration Bcad hexagonal ferrite magnetic field of calender composition time frequency Strength Kind diameter powder) Time strength roll (part) (second) (kHz) (W) Reference None None None None None  80° C. 0.2 None None None Example 1 Reference None None None None None  95° C. 0.2 None None None Example 2 Comparative None None None None None 100° C. 0.2 None None None Example 1 Comparative None None None None None 110° C. 0.2 None None None Example 2 Comparative Diamond 500 nm 10 times 1 h 0.15T 100° C. 0.2 None None None Example 3 Comparative Diamond 500 nm 10 times 1 h 0.15T 100° C. 0.0 0.8 30 100 Example 4 Comparative Diamond 500 nm 10 times 1 h 0.15T 100° C. 1.2 0.8 30 100 Example 5 Comparative Diamond 500 nm 10 times 1 h 0.15T 100° C. 0.2 0.8 30 100 Example 6 Comparative None None None None None 100° C. 0.2 0.8 30 100 Example 7 Comparative None None None None 0.15T 100° C. 0.2 0.8 30 100 Example 8 Comparative None None None None 0.30T 100° C. 0.2 0.8 30 100 Example 9 Comparative Diamond 500 nm 10 times 1 h 1.00T 100° C. 0.2 0.8 30 100 Example 10 Comparative Diamond 500 nm 10 times 1 h None 100° C. 0.2 0.8 30 100 Example 11 Example 1 Diamond 500 nm 10 times 1 h 0.15T 100° C. 0.2 0.8 30 100 Example 2 Diamond 500 nm 10 times 1 h 0.15T 100° C. 0.5 0.8 30 100 Example 3 Diamond 500 nm 10 times 1 h 0.15T 100° C. 1.0 0.8 30 100 Example 4 Diamond 500 nm 10 times 1 h 0.20T 100° C. 0.2 0.8 30 100 Example 5 Diamond 500 nm 10 times 1 h 0.30T 100° C. 0.2 0.8 30 100 Example 6 Diamond 500 nm 10 times 1 h 0.50T 100° C. 0.2 0.8 30 100 Example 7 Diamond 500 nm 20 times 1 h 0.15T 100° C. 0.2 0.8 30 100 Example 8 Diamond 500 nm 10 times 1 h 0.30T 110° C. 0.2 0.8 30 100 Example 9 Diamond 500 nm 10 times 1 h 0.30T 110° C. 0.2 2.0 30 100 Example 10 Diamond 500 nm 10 times 1 h 0.30T 110° C. 0.2 0.8 30 100 Example 11 Diamond 500 nm 10 times 1 h 0.30T 110° C. 0.2 2.0 30 100 Ratio of Back coating Missing pulse inorganic layer XRD intensity Magnetic layer generation powder/carbon S_(after)-S_(before) FWHM_(before) FWHM_(after) ratio Vertical surface frequency black (nm) (nm) (nm) Int (110)/Int (114) squareness ratio roughness Ra (times/m) Reference 80/20 1.5 13.0 8.0 0.2 0.55 2.5 nm 3 Example 1 Reference 80/20 1.6 13.1 8.0 0.2 0.55 2.0 nm 4 Example 2 Comparative 80/20 1.5 13.1 7.9 0.2 0.55 1.8 nm 13 Example 1 Comparative 80/20 1.6 13.0 8.0 0.2 0.55 1.5 nm 18 Example 2 Comparative 80/20 1.6 13.0 8.0 0.5 0.70 1.8 nm 12 Example 3 Comparative 80/20 0.0 7.9 8.0 0.5 0.70 1.8 nm 12 Example 4 Comparative 80/20 9.6 8.0 8.0 0.5 0.70 1.8 nm 12 Example 5 Comparative  0/100 1.7 7.9 12.0 0.5 0.70 1.8 nm 12 Example 6 Comparative 80/20 1.6 8.1 8.0 0.2 0.55 1.8 nm 12 Example 7 Comparative 80/20 1.6 8.0 7.9 3.8 0.63 1.8 nm 12 Example 8 Comparative 80/20 1.7 8.0 7.9 5.0 0.75 1.8 nm 12 Example 9 Comparative 80/20 1.6 8.1 8.0 6.1 0.90 1.8 nm 12 Example 10 Comparative 80/20 1.5 8.0 8.0 0.3 0.66 1.8 nm 12 Example 11 Example 1 80/20 1.6 7.9 8.0 0.5 0.70 1.8 nm 5 Example 2 80/20 4.0 7.9 8.0 0.5 0.70 1.8 nm 5 Example 3 80/20 8.0 7.9 8.0 0.5 0.70 1.8 nm 5 Example 4 80/20 1.6 8.0 8.0 1.5 0.75 1.8 nm 4 Example 5 80/20 1.7 8.0 8.1 2.3 0.80 1.8 nm 4 Example 6 80/20 1.6 8.1 8.0 4.0 0.85 1.8 nm 4 Example 7 80/20 1.6 8.0 7.9 0.7 0.83 1.8 nm 5 Example 8 80/20 1.5 8.1 8.0 2.3 0.80 1.5 nm 6 Example 9 80/20 1.6 2.0 8.0 2.3 0.80 1.5 nm 3 Example 10 100/0  1.6 8.0 2.0 2.3 0.80 1.5 nm 3 Example 11 100/0  1.6 2.0 2.0 2.3 0.80 1.5 nm 3

With the comparison of Reference Examples 1 and 2 and Comparative Examples 1 to 11, it is possible to confirm that the edge damage significantly occurs, in a magnetic tape having the magnetic layer surface roughness Ra equal to or smaller than 1.8 nm (Comparative Examples 1 to 11).

In addition, with the comparison of Examples 1 to 11 and Comparative Examples 1 to 11, it is possible to confirm that the occurrence of the edge damage in the magnetic tape having the magnetic layer surface roughness Ra equal to or smaller than 1.8 nm can be prevented by setting the XRD intensity ratio, the vertical squareness ratio, the FWHM_(before) and the FWHM_(after) of the back coating layer, and the difference (S_(after)−S_(before)) to be in the respective ranges described above.

One aspect of the invention can be effective in the technical fields of magnetic tapes such as back-up tapes. 

What is claimed is:
 1. A magnetic tape comprising: a non-magnetic support; a magnetic layer including ferromagnetic powder, non-magnetic powder, and a binding agent on one surface side of the non-magnetic support; and a back coating layer including non-magnetic powder and a binding agent on the other surface side of the non-magnetic support, wherein the ferromagnetic powder is ferromagnetic hexagonal ferrite powder, a center line average surface roughness Ra measured regarding a surface of the magnetic layer is equal to or smaller than 1.8 nm, an intensity ratio Int(110)/Int(114) of a peak intensity Int(110) of a diffraction peak of a (110) plane with respect to a peak intensity Int(114) of a diffraction peak of a (114) plane of a hexagonal ferrite crystal structure obtained by an X-ray diffraction analysis of the magnetic layer by using an In-Plane method is 0.5 to 4.0, a vertical squareness ratio of the magnetic tape is 0.65 to 1.00, the back coating layer includes fatty acid ester, a full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer before performing a vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 10.0 nm, a full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 10.0 nm, and a difference S_(after)−S_(before) between a spacing S_(after) measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape and a spacing S_(before) measured by optical interferometry regarding the surface of the back coating layer before performing the vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 8.0 nm.
 2. The magnetic tape according to claim 1, wherein the center line average surface roughness Ra measured regarding the surface of the magnetic layer is 1.2 nm to 1.8 nm.
 3. The magnetic tape according to claim 1, wherein the vertical squareness ratio is 0.65 to 0.90.
 4. The magnetic tape according to claim 1, wherein the full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer before performing the vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 8.5 nm.
 5. The magnetic tape according to claim 1, wherein the full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer after performing the vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 8.5 nm.
 6. The magnetic tape according to claim 1, wherein the difference S_(after)−S_(before) is greater than 0 nm and equal to or smaller than 6.0 nm.
 7. The magnetic tape according to claim 1, wherein the center line average surface roughness Ra measured regarding the surface of the magnetic layer is 1.2 mu to 1.8 nm, the vertical squareness ratio is 0.65 to 0.90, the full width at half maximum of spacing distribution measured by optical interferometry regarding the surface of the back coating layer before performing the vacuum heating with respect to the magnetic tape is greater than 0 nm and equal to or smaller than 8.5 mu, and the difference S_(after)−S_(before) is greater than 0 nm and equal to or smaller than 6.0 nm.
 8. The magnetic tape according to claim 1, wherein the non-magnetic powder included in the back coating layer is one or more kinds of non-magnetic powder selected from the group consisting of inorganic powder and carbon black.
 9. The magnetic tape according to claim 8, wherein a content of the inorganic powder in the back coating layer is greater than 50.0 parts by mass and equal to or smaller than 100.0 parts by mass with respect to 100.0 parts by mass of a total amount of the non-magnetic powder included in the back coating layer.
 10. The magnetic tape according to claim 1, further comprising: a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer. 